First of all, OMG Zombies! is a puzzle game. A bit crazy, random-generated, puzzle game where winning is based on a well-calculated and timed click and the ensuing chaos. Your success is ultimately dictated on that crucial first click and statistics. Oh, and there are a lot of zombies too.
That tickles your interest? Basically, in each level you find yourself somewhere high, surrounded by zombies and with limited ammunition. Each zombie has a special power triggered on their death, like explosion or acid pool or shooting a weapon. With just a few bullets, you have to kill as many zombies as possible, preferably all of them.
This is difficult at first. Then you begin to see patterns within the horde and also accumulate money from winning levels, so it gets easier as you can upgrade your weapon and the zombies. It's left unclear how the zombies can be made stronger, but somehow that happens when you pump enough money in them. Whatever really, as it surely is surprisingly addictive. Several times I decided that the next level would be the last but then kept on playing regardless.
There's also a story. No, it's not very detailed or good, but it explains where the zombies came from and why you find yourself in this dire situation. Also, it's told in an artsy comic manner, so it's cool and it works well for this game. The whole game is drawn in greyscale, except the blood that's (you guessed it) red and everywhere. The unique style works well. Sounds are repetitive and uninteresting, but the cacophony of them can be amusing when you pull a great combo.
For me it lasted 8 hours for the first playthrough (100% completion until first prestige). That brings me to the one thing that kind of bugs me: there's a bad case of "that achievement" in this game, as to earn the last achievement you have to (you get to?) beat the game completely (100%) 20 times. That's going to take a while. I'd recommend the game regardless as it's cheap and fun. It's also a good game to go to when you're waiting for something but only got a few minutes to spare as it's fast to play through just a few levels.